In order to impart antibacterial activity to various kinds of synthetic resin molded articles, such as fibers and the like, it has been considered that a metal having antibacterial activity is added to a synthetic resin material. For example, there has been such a method in that a metal or metallic compound having antibacterial activity is added directly to a synthetic resin material. However, due to a significant difference in physical property between the metal or metallic compound and the synthetic resin material, the synthetic resin material is largely influenced thereby to restrict the range of utilization thereof, and furthermore, there is such a possibility that the metal or metallic compound is released from the synthetic resin molded article during use to cause influence on durability of the antibacterial activity and to cause unexpected adverse side effect with the metal or metallic compound thus released.
In order to solve the problem, JP-B-63-54013 and JP-A-63-175117 disclose that antibacterial zeolite obtained by carrying an antibacterial metal on zeolite by ion exchange is added to fibers or a fiber material.
JP-A-2-180270, JP-A-3-218765 and JP-A-5-154 disclose an antibacterial composition obtained by carrying an antibacterial metal and a metallic ion thereof on hydroxyapatite, which is most frequently used among calcium phosphate materials.
However, in the case where powder of the antibacterial zeolite or the antibacterial hydroxyapatite having an antibacterial metallic ion carried thereon is added directly to, for example, a raw material polymer for fibers, to prevent propagation of microorganisms, the zeolite or hydroxyapatite powder is liable to be aggregated among particles and is difficult to be dispersed, and therefore, the antibacterial agent is difficult to be present uniformly on the surface of fibers to fail to obtain good antibacterial activity. Accordingly, unevenness in quality of antibacterial activity is liable to occur when a small amount of the antibacterial agent is used.
In view of the above, it has been attempted upon mixing antibacterial ceramic powder with a raw material polymer for various kinds of synthetic resin molded articles that, as the first step, a master batch containing the antibacterial agent in a high concentration is prepared, and as the second step, the master batch is added to the raw material polymer for molded articles to obtain a prescribed concentration of the antibacterial agent, so as to attain uniform mixing.
In the case where the aforementioned production process is carried out, it is necessary that the master batch mixed with the antibacterial ceramic powder and the raw material polymer for molded articles are simultaneously melted. Therefore, it is preferred that the master batch has a melting point and physical property that are completely same as those of the raw material polymer for the synthetic resin composition constituting the molded articles. In order to satisfy the requirement, it is necessary that the base material of the master batch having the antibacterial agent mixed therein is the same as the polymer, and the particle diameter of the master batch is reduced.
However, in order to obtain a finely particulate master batch, it is necessary that the master batch having been once produced is pulverized by sufficiently cooled with a freezing medium and then subjected to some particular process steps, such as homogenization of the particle diameter and the like step, and therefore, it is disadvantageous in production time and cost. Furthermore, while it has been proposed that a polymer having a low melting point, such as polyolefin wax and the like, is used as the base material of the master batch, there is a significantly large difference in viscosity between the master batch base material and the basic material resin at the production temperature of molded articles, whereby the master batch suffers segregation to cause such a problem that it cannot be uniformly mixed in the basic material resin.